Chris,
Referring back to the photo you posted of your magdyno - that is a magdyno for the later mark series machines. The original magdyno for the prewar machines had a thinner front cover and no side lugs for bolting up to the timing chest. There doesn't seem to be many detailed photos of prewar Douglas magdynos, but I would assume they were basically the same for 'Blue Chief', 'Wessex', 'Powerflow', 'Speed Special' and 'Endeavour' models - just the 'Powerflow' and 'Wessex' models were listed as having a 'quickly removable' magdyno - so the drive is probably peculiar to those 2 models! I have, however, managed to find an illustration of the magdyno on an 'Endeavour' and it shows a very thin front cover with a deep extension to house the drive. I remember a few years back, an LDMCC member had the same problem with a mag for a 'Speed Special' - he turned up the extension and had it laser welded to a standard Lucas MO1 magneto front cover - that turned out very successful, as the laser welding didn't cause any distortion.
Regards,
Eddie.